262 



and radiating ; the former contracting, the latter dilating the pupil. 

 Some horses have a white, unpigmented iris, and are called "wall- 

 eyed," or " albinos." 



624. The Corpora Nigra {Plate XXXVI. , A. 7.) are the small 

 black bodies, of a globular shape, to be observed in front of the 

 papillary openmg, and generally seen on the upper fringe of the iris; 

 their use is supposed to be to modify the rays of light. 



625. The Humours of the eye are— the aqueous, the crystalliue, and 

 the vitreous; they act as refractive media. The aqueous {Plate 

 XXXVL. B. 6. and 12.) is in front, between the interior surface of 

 the cornea, and the crystalhne lens. It is composed chiefly of water, 

 and has an alkaline reaction. The crystalline lens {Plate XXXVL, 

 B. 8.) is bi-convex, more flattened in front than behind. It is 

 made up of concentric layers, like an onion, and is enclosed in a 

 capsule. It is situated behind the iris, having the aqueous humour 

 in front, and the vitreous behind, and is held in position by a 

 suspensory ligament. It is transparent, its use being to concentrate 

 the rays of light to a focus. The vitreous humour {Plate XXXVL, 

 B. 7., 7.) occupies the posterior chamber, which constitutes about 

 four-fifths of the interior of the sac. It is an albuminous semi-fluid, 

 of a jelly-like consistency, and is enclosed in a structure termed the 

 hyaloid niemhyane. 



626. The Retina {Plate XXXVL, B. 5.) is the expansion of the 

 optic nerve. It is composed of several layers, consisting of rods, 

 cones, &c., and is the essential part of the organ of vision. The 

 optic nerve {Plate XXXVL, B. 16., 16.) enters the eyeball, in 

 company with the opthalmic artery and vein, at the back of the eye. 



627. The muscles of the eye are— the Retractor, Abductor, 

 Adductor, Depressor, Levator, and superior, and inferior Obliques. 

 These muscles turn the eye in the various directions required. 



